In machines for producing a web of paper or paper board, it is common to direct an endless rope or a pair of endless ropes around the end portions of a series of rolls, such as the rolls of the dryer section or a set of calender rolls, for gripping and threading through the rolls the leading end portion of a web, usually referred to as a "tail". When a single rope is used, it is directed twice around the rolls so that two adjacent ropes extend through the rolls and are effective to pinch the tail and thread it around the rolls. The rope may have a diameter ranging from 1/4" to 5/8" and is usually made of spun nylon filaments or of natural fibers. The rope stretches over a period of use, especially when the rope is driven at a higher speed, for example, over several thousand feet per minute. The ropes can stretch from 7% to 10% within a few days or within a few months depending upon the speed of the papermaking machinery. In the dryer section, the length of the endless rope is frequently between 200 feet and 400 feet so that a 7% to 10% stretch requires substantial take up in the rope in order to maintain the desired rope tension.
In prior art rope stretchers, various types of elongated Frames have been fabricated from metal rods and bars of various shapes, and the fabricated steel frames support movable carriages. Flexible cables extend from the carriages around corresponding stationary guide pulleys and also around movable pulleys supported by the extendable piston rods of air cylinders. In such a rope stretcher, it is desirable to simplify its construction and to reduce the number of moving parts so that servicing is minimized or substantially eliminated thereby avoiding down time of the papermaking machine in the event the rope stretcher is not properly serviced. It is also desirable to maintain constant tension in the rope during threading operation to provide dependable and efficient threading of a web tail and to obtain maximum service life from the rope. In addition, it is highly desirable to provide a more compact rope stretcher having a lighter weight and greater strength as well as a rope stretcher which may be installed in any position or orientation on papermaking machinery.
The above mentioned application Ser. No. 07/783,425 discloses an improved rope stretcher which provides all of the desirable features and advantages mentioned above, and which is especially simple, economical and dependable in construction. That rope stretcher further eliminates the need to fabricate an elongated track and the use of cable and pulley systems, and it may be easily constructed in various sizes and lengths depending on the length and stretch of the rope and the desired tension in the rope.
In one embodiment of a rope stretcher disclosed in the above mentioned application, an elongated precision track is formed by an H-beam having parallel flanges integrally connected by a center web. Opposite end portions of the beam are supported by mounting brackets, and one of the flanges supports a pair of adjacent parallel carriage plates by a set of sealed anti-friction guide wheels which positively engage opposite longitudinal edge portions of the flange.
Each of the carriage plates supports a rope sheave for free rotation by a sealed anti-friction bearing and is directly connected by a link member to the end portion of a piston rod projecting from a corresponding elongated fluid or air cylinder mounted on the web of the beam. A pair of proximity sensors are mounted on opposite end portions of the beam to detect the presence of the carriages and actuate a signal or alarm when the stretch in the rope is approaching a limit. A longitudinally extending cover extends over the beam and carriages to protect the carriages and fluid cylinders.
When the above described rope stretcher is mounted at an angle to the horizontal, the weight of the cylinder piston, piston rod, carriage, sheave and connecting parts which move downwardly to tension the rope add to the force produced by the corresponding cylinder. The weight of the corresponding parts which move upwardly to tension the rope counteracts that cylinder's force. This creates a situation causing improper and variable rope tension depending on whether the downward moving sheave has reached the end of its stroke.